Liquid crystal on silicon (LCoS) devices are of great interest. They provide high resolution, high brightness and high contrast and are utilized in high-definition television, near-eye viewing devices, and projectors. Most of these devices utilize liquid crystal polarization rotation electro-optical effects to modulate the output of light. A drawback of these devices is that their pixel density cannot be very high because of the need for the devices to avoid the diffraction effect and fringe field effect. Yet another limitation on pixel density is that the light transmissions through these devices become lower because of the significant increase in the corresponding of non-active areas. These non-active areas are also referred to as the inter pixel regions. In other words, the regions between the pixel boundaries, although minimal, reduce the active area for light transmission.
The concept of a rectangular diffraction grating filled with liquid crystal has been reported in U.S. Pat. No. 4,729,640 by Sakata et al. Although effective in its stated purpose, the grating of Sakata does not provide optimum light transmission. Other groups have used high resolution LCDs to display holograms with 30 μm width pixels, providing extremely limited holograph image quality. However, it is unsure whether this device can be easily reproduced. Indeed, with a 60 μm size, over 350 orders are transmitted through a grating, each a half degree apart. The 0th order could still be modulated with such a device, but baffling of the stray light in non-zero orders is believed to be a difficult task.
The phase profile for an ideal high efficiency diffraction grating is shown in FIG. 1A. It will be appreciated that the ideal waveform is sharp and well delineated between two phase states to provide a square-like phase profile. However, the phase profile for a patterned electric field liquid crystal grating typically looks like sinusoid wave as shown in FIG. 1B. It will be appreciated that such a wave form does not provide a high-diffraction efficiency and therefore the performance characteristics of the grating are much less than desired.
Accordingly, there is a need in the art to produce a square-like phase grating to improve the high-diffraction efficiency.